I realize this post is over 2 months old and you might have figured it out already, but I know how frustrating these things can be.  

I have basically the same setup that I have been working on also. If you are wanting the buttons to affect anything while running raspbmc, instead of adding keybindings to open box (which work on a regular raspbian image), you need to update xbmc's key map.

XBMC's key map is what converts keyboard presses to useful commands. So spacebar to play pause, backspace to go back, "x" to stop, ect... If you google "xmbc keymap" the first link will explain everything.

I am not running raspbmc so I am not possitive, but I believe the keymap is located in /home/pi/.xbmc/userdata/keymaps/keyboard.xml

In the keyboard.xml file you will need to insert something like this:
<keymap>
  <global>
    <keyboard>
        <n mod="ctrl,alt">Play</n>
        <x mod="ctrl,alt">Stop</x>
        <m mod="ctrl,alt">Back</m>
        <key id="389184" mod="ctrl,alt">VolumeUp</key>
        <key id="61596">VolumeDown</key>
    </keyboard>
  </global>
</keymap>


This will work assuming that your udev rule is working. If it is working the buttons will light up weather or not they send a command to your system.

I assume you are wanting to control the volume of your carpc with the buttons. If this is the case I recommend controlling the volume in an xbmc skin instead of the buttons as they are too slow and take too long to adjust the volume level. Then you can use the buttons for other useful things.

I can't post links as I am a new member, but search for "Andrei Istodorescu Car PC project(August 2013 update)". Andrei has a blog that has a welth of information and his post on Aug 2013 has an image that he compiled himself with a skin that has volume control built into it.

Hope that helps!

Welcome! I'm new here also and am excited to learn all about this world of bitcoin. It makes currency and economics much more interesting that what I learned in high school!